Hoag was born in Newburyport, Massachusetts. As a teen, Hoag attended Walnut Hill School of Performing Arts in Natick, Massachusetts where she concentrated on acting. In 1988, she married to actor Vince Grant.
Hoag has acted professionally since 1986. From 1986 to 1987 she was series regular as Charlotte 'Lotty' Bates Alden in the ABC daytime soap opera Loving. In 1988, Hoag began her career in primetime television, and in next year won female lead role on CBS comedy series Wolf. . The series was canceled after single season. In 1990 she starred in films A Matter of Degrees and Cadillac Man, and later was cast as April O'Neil in the first Teenage Mutant Ninja Turtles movie.
Hoag is most well known for her role as April O'Neil in the first Teenage Mutant Ninja Turtles film. The film turned out to be a huge success at the box office, eventually making over $135 million in North America, and over $66 million outside North America for a worldwide total of over $200 million, making it the ninth highest grossing film of 1990 worldwide. After Teenage Mutant Ninja Turtles fame, Hoag starred in number of not picked up as a series pilots, and appeared in several television films, including Fine Things by Danielle Steel, and Switched at Birth opposite Bonnie Bedelia.
Hoag starred as Gwen Cromwell Piper in the Disney Channel Halloweentown television films series: Halloweentown (1998), Halloweentown II: Kalabar's Revenge (2001), Halloweentown High (2004) and Return to Halloweentown (2006). She also appeared in the films Armageddon (1998), Flying By (2009), I Am Number Four (2011) and Hitchcock (2012).
Hoag has made over 60 guest appearances on television shows, including Quantum Leap, Melrose Place, Roseanne, The Nanny, Murder, She Wrote, The X-Files, Six Feet Under, Ghost Whisperer, NYPD Blue, Grey's Anatomy, Private Practice, CSI: NY, Criminal Minds, Sons of Anarchy, Castle, The Middle, Grimm and among other notable television series. From 2006 to 2011, Hoag also appeared as Cindy Price on the HBO drama series Big Love.
In 2012, Hoag was cast in recurring role in the ABC drama series Nashville created by Academy Award winner Callie Khouri. She plays the poised and driven Tandy Hampton, daughter and protégé of Lamar Wyatt. She referees sister Rayna (Connie Britton) and Lamar (Powers Boothe) contentious relationship, trying to calm the waters.